Three-dimensional NAND non-volatile memory devices with buried word line selectors

ABSTRACT

Three-dimensional NAND stacked memory devices include a stack including alternating word line and dielectric layers and a plurality of NAND strings of memory cells formed in memory holes which extend through the layers. Each memory cell includes a control gate formed by one of the word line layers, and multiple selector devices, each selector device coupled to an end of a corresponding NAND string. The NAND strings are disposed above a substrate, and the selector devices are disposed in the substrate.

REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/869,578, filed on Aug. 23, 2013, which is incorporatedherein by reference in its entirety for all purposes.

BACKGROUND

The present technology relates to three-dimensional (3D) non-volatilememory devices. In particular, the present technology relates to 3D NANDnon-volatile memory devices with buried word line selectors.

Recently, ultra-high density storage devices have been proposed using a3D stacked memory structure. For example, a 3D NAND stacked non-volatilememory device can be formed from an array of alternating conductive anddielectric layers. A memory hole is drilled in the layers to define manymemory layers simultaneously. A NAND string is then formed by fillingthe memory hole with appropriate materials. Control gates of the memorycells are provided by the conductive layers. Examples of such devicesinclude Bit Cost Scalable (BiCS) memory, Terabit Cell Array Transistor(TCAT) and Vertical NAND (V-NAND).

However, various challenges are presented in designing and operatingsuch memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Like-numbered elements refer to common components in the differentfigures.

FIGS. 1A and 1B are perspective views of previously known 3D NANDstacked non-volatile memory devices.

FIG. 2A is a perspective view of an embodiment of a 3D NAND non-volatilememory device.

FIG. 2B illustrates a top view of the 3D NAND non-volatile memory deviceof FIG. 2A.

FIGS. 2C-2E illustrate various cross-sectional views of the 3D NANDnon-volatile memory device of FIGS. 2A-2B.

FIGS. 2F-2I illustrate close-up cross-sectional views of portions of the3D NAND non-volatile memory device of FIGS. 2C-2E.

FIG. 3A is a perspective view of another embodiment of a 3D NANDnon-volatile memory device.

FIG. 3B illustrates a top view of the 3D NAND non-volatile memory deviceof FIG. 3A.

FIGS. 3C-3E illustrate various cross-sectional views of the 3D NANDnon-volatile memory device of FIGS. 3A-3B.

FIGS. 3F-3I illustrate close-up cross-sectional views of portions of the3D NAND non-volatile memory device of FIGS. 3C-3E.

FIG. 4A is a block diagram of an integrated 3D NAND and DRAM.

FIG. 4B is a cross-sectional view of the integrated 3D NAND and DRAM ofFIG. 4A.

FIG. 5A is a perspective view of a 3D stacked non-volatile memorydevice.

FIG. 5B is a functional block diagram of the 3D stacked non-volatilememory device of FIG. 4A.

DETAILED DESCRIPTION

A 3D NAND stacked non-volatile memory device may include an array ofalternating conductive and dielectric layers disposed above a substrate.A memory hole is drilled in the layers to define many memory layerssimultaneously. A NAND string is then formed by filling the memory holewith appropriate materials. Control gates of the memory cells areprovided by the conductive layers. Each NAND string has a first “drain”end coupled via a drain-side select gate transistor (“SGD”) to a bitline, and a second “source” end coupled via a source-side select gatetransistor (“SGS”) to a common source conductor. SGD and SGS may be usedto selectively couple the drain and source ends, respectively, of a NANDstring to the bit line and source line, respectively.

FIG. 1A illustrates a previously-known TCAT array 10 a, and FIG. 1Billustrates a previously-known BiCS array 10 b. TCAT array 10 a includesa NAND string 12 a disposed above a substrate 14 a. NAND string 12 a hasa drain end 16 a coupled via SGD 18 a to a bit line 20 a, and a sourceend 22 a coupled via SGS 24 a to a source line 26 a. BiCS array 10 bincludes a NAND string 12 b disposed above a substrate 14 b. NAND string12 b has a drain end 16 b coupled via SGD 18 b to a bit line 20 b, and asource end 22 b coupled via SGS 24 b to a source line 26 b.

In each of the previously known 3D NAND memory arrays 10 a and 10 b,select gates SGD 16 a and SGS 20 a, and SGD 16 b and SGS 20 b areimplemented above substrates 14 a and 14 b, respectively. In addition,in such previously known 3D NAND memory arrays 10 a and 10 b, SGD 16 aand SGS 20 a, and SGD 16 b and SGS 20 b consume a significant amount ofarea, and may limit the ability to achieve high-density memory arrays.

Unlike previously known 3D NAND non-volatile memory devices, such asthose illustrated in FIGS. 1A and 1B, 3D NAND non-volatile memorydevices are described that include select gate transistors (SGD or SGS)disposed in the substrate below the NAND strings. In particular, 3D NANDmemory arrays are described that include buried word lines as selectordevices of select gate transistors (SGD or SGS). Without wanting to bebound by any particular theory, such buried word line selector devicesmay be used to form high density 3D NAND non-volatile memory devices.

An embodiment of a 3D NAND non-volatile memory device with buried wordline selectors is shown in FIGS. 2A-2I. FIG. 2A illustrates aperspective view of a portion of 3D NAND non-volatile memory device 100a. FIG. 2B illustrates a top view of 3D NAND non-volatile memory device100 a, FIGS. 2C-2E illustrate various cross-sectional views of 3D NANDnon-volatile memory device 100 a, and FIGS. 2F-2I illustrate close-upcross-sectional views of portions of 3D NAND non-volatile memory device100 a. 3D NAND non-volatile memory device 100 a includes bit lines BL0,BL1, . . . , BL5, vertical NAND strings N₀₀, N₀₁, . . . , N₅₅, andsource lines SL0, SL1, . . . , SL5 disposed above a surface 110 of asubstrate 112, and conductors BWL0, BWL1, . . . , BWL5 disposed belowsurface 110 of substrate 112. In an embodiment, source lines SL0, SL1, .. . , SL5 are connected to one another at a common source node (notshown).

Substrate 112 also includes active regions AR0, AR1, . . . , AR5. In anembodiment, each of active regions AR0, AR1, . . . , AR5 are segmentedby isolation trenches (e.g., SiO₂) 114. In this embodiment, bit linesBL0, BL1, . . . , BL5 are disposed above and are parallel to activeregions AR0, AR1, . . . , AR5, and source lines SL0, SL1, . . . , SL5are disposed above and are parallel to conductors BWL0, BWL1, . . . ,BWL5. In addition, bit lines BL0, BL1, . . . , BL5 are perpendicular toconductors BWL0, BWL1, . . . , BWL5. In other embodiments, otherconfigurations are possible. For example, active regions AR0, AR1, . . ., AR5 may be disposed at an angle relative to bit lines BL0, BL1, . . ., BL5, conductors BWL0, BWL1, . . . , BWL5, and source lines SL0, SL1, .. . , SL5.

Each of vertical NAND strings N₀₀, N₀₁, . . . , N₅₅ includes columnsC₀₀, C₀₁, . . . , C₅₅, respectively, and alternating conductive layersand dielectric layers. Conductive layers include select gate layers SG0,SG1, . . . , SG5 and word line layers WL0, WL1, . . . , WL7. Forexample, vertical NAND string N₀₀ includes select gate layer SG0 andword line layers WL0, WL1, . . . , WL7, each separated from adjacentconductive layers by dielectric material layers (e.g., SiO₂ or otherdielectric material) (not shown in FIGS. 2C-2E). Likewise, NAND stringN₁₀ includes select gate layer SG1 and word line layers WL0, WL1, . . ., WL7, each separated from adjacent conductive layers by dielectricmaterial layers (not shown in FIGS. 2C-2E). In an embodiment, word linelayers WL0, WL1, . . . , WL7 are perpendicular to bit lines BL0, BL1, .. . , BL5.

FIGS. 2F-2G depict a close-up view of region 116 a of column C₀₀ of FIG.2C, showing a drain side select gate transistor SGD₀₀ and a memory cellMC₀₇. Region 116 a shows portions of dielectric material layers D0, D1and D2, select gate layer SG0, and word line layer WL7. Each of columnsC₀₀, C₀₁, . . . , C₅₅, includes a number of layers which are depositedalong the sidewalls of the column. These layers may includeoxide-nitride-oxide and polysilicon layers which may be deposited (e.g.,using atomic layer deposition or other technique). For example, a blockoxide (BOX) can be deposited as layer 118, a nitride such as SiN as acharge trapping layer (CTL) can be deposited as layer 120, a tunneloxide (TNL) can be deposited as layer 122, a polysilicon body or channel(CH) can be deposited as layer 124, and a core filler dielectric can bedeposited as region 126. Additional memory cells are similarly formedthroughout columns C₀₀, C₀₁, . . . , C₅₅. In the embodiment shown inFIGS. 2A-2G, each of columns C₀₀, C₀₁, . . . , C₅₅ has a hollow cylindershape. In other embodiments, each of columns C₀₀, C₀₁, . . . , C₅₅ mayhave a solid, rod shape.

Referring again to FIGS. 2A-2E, conductors BWL0, BWL1, . . . , BWL5 aredisposed in dielectric trenches 130 (e.g., SiO₂ shallow trench isolation(STI) trenches) formed in a doped well 132 (e.g., p-well) in substrate112. As used herein, conductors BWL0, BWL1, . . . , BWL5 will bereferred to as “buried word lines” BWL0, BWL1, . . . , BWL. Persons ofordinary skill in the art will understand that three-dimensional NANDdevice 100 a may include more or fewer bit lines BL0, BL1, . . . , BL5,vertical NAND strings N₀₀, N₀₁, . . . , N₅₅, source lines SL0, SL1, . .. , SL5, and buried word lines BWL0, BWL1, . . . , BWL5 than depicted inFIGS. 2A-2E.

Select gate layers SG0, SG1, . . . , SG5 form conductive paths tocontrol select gate transistors of NAND strings N₀₀, N₀₁, . . . , N₅₅,and word lines WL0, WL1, . . . , WL7 form conductive paths to controlgates of the memory cells at the layer. Conductive layers SG0, SG1, . .. , SG5 and WL0, WL1, . . . , WL7 may include conductive orsemiconductor material, such as doped polysilicon or a metal, such astungsten, copper, aluminum, tantalum, titanium, cobalt, titanium nitrideor alloys, or other conductive material. Although vertical NAND stringsN₀₀, N₀₁, . . . , N₅₅ are shown each including eight word line layers,person of ordinary skill in the art will understand that more or fewerthan eight word line layers may be used.

Select gate layers SG0, SG1, . . . , SG5 may be used to selectivelycouple the “drain” ends of a NAND strings N₀₀, N₀₁, . . . , N₅₅ to oneof bit lines BL0, BL1, . . . , BL5. For example, select gate layer SG0may be used to selectively couple NAND strings N₀₀, N₀₁, N₀₂, N₀₃, N₀₄and N₀₅ to bit lines BL0, BL1, BL2, BL3, BL4 and BL5, respectively.Likewise, select gate layer SG4 may be used to selectively couple NANDstrings N₄₀, N₄₁, N₄₂, N₄₃, N₄₄ and Nay to bit lines BL0, BL1, BL2, BL3,BL4 and BL5, respectively. Bit lines BL0, BL1, BL2, BL3, BL4 and BL5 maybe tungsten or other conductive material.

The “source” ends of NAND strings N₀₀, N₀₁, . . . , N₅₅ are coupled toactive regions AR0, AR1, . . . , AR5 via NAND string contacts 134 (e.g.,highly doped polysilicon plugs, such as n+ polysilicon plugs). Inparticular, NAND strings N₀₀, N₁₀, N₂₀, N₃₀, N₄₀ and N₅₀ are coupled toactive region AR0, NAND strings N₀₁, N₁₁, N₂₁, N₃₁, N₄₁ and N₅₁ arecoupled to active region AR1, NAND strings N₀₂, N₁₂, N₂₂, N₃₂, N₄₂ andN₅₂ are coupled to active region AR2, NAND strings N₀₃, N₁₃, N₂₃, N₃₃,N₄₃ and N₅₃ are coupled to active region AR3, NAND strings N₀₄, N₁₄,N₂₄, N₃₄, N₄₄ and N₅₄ are coupled to active region AR4, and NAND stringsN₀₅, N₁₅, N₂₅, N₃₅, N₄₅ and N₅₅ are coupled to active region AR5. NANDstring contacts 134 are separated from one another and from source linesSL0, SL1, . . . , SL5 by dielectric material 136 (e.g., SiO₂ or otherdielectric material).

Source lines SL0, SL1, . . . , SL5 may include a stack of materiallayers, such as shown in FIGS. 2C-2D. In an embodiment, each of sourcelines SL0, SL1, . . . , SL5 include a multi-layer stack of anon-conductive material layer 140 (e.g., an etch stop or hard maskmaterial), a first conductive material layer 142, and a secondconductive material layer 144. For example, non-conductive materiallayer 140 may be a nitride, first conductive material layer 142 may betungsten (W), and second conductive material layer 144 may be highlydoped polysilicon (e.g., n+ poly). In an embodiment, non-conductivematerial layer 140, first conductive material layer 142, and secondconductive material layer 144 may be formed on a gate contact layer of aCMOS process. Other materials and processes may be used to form sourcelines SL0, SL1, . . . , SL5.

Source lines SL0, SL1, . . . , SL5 form multiple electrical contactswith p-well 132 where source lines SL0, SL1, . . . , SL5 overlap each ofactive regions AR0, AR1, . . . , AR5. For example, referring to FIG. 2C,source line SL0 forms an electrical contact to p-well 132 at a region150 where source line SL0 overlaps active region AR0, and source lineSL1 forms an electrical contact to p-well 132 at a region 152 wheresource line SL1 overlaps active region AR1.

Buried word lines BWL0, BWL1, . . . , BWL5 may be used to selectivelycouple the “source” ends of NAND strings N₀₀, N₀₁, . . . , N₅₅ to one ofsource lines SL0, SL1, . . . , SL5. In an embodiment, buried word lineBWL0 may be used to selectively couple source ends of NAND strings N₀₀,N₀₁, N₀₂, N₀₃, N₀₄ and N₀₅ to source line SL0; buried word line BWL1 maybe used to selectively couple source ends of NAND strings N₁₀, N₁₁, N₁₂,N₁₃, N₁₄ and N₁₅ to source line SL1; buried word line BWL2 may be usedto selectively couple source ends of NAND strings N₂₀, N₂₁, N₂₂, N₂₃,N₂₄ and N₂₅ to source line SL2; buried word line BWL3 may be used toselectively couple source ends of NAND strings N₃₀, N₃₁, N₃₂, N₃₃, N₃₄and N₃₅ to source line SL3; buried word line BWL4 may be used toselectively couple source ends of NAND strings N₄₀, N₄₁, N₄₂, N₄₃, N₄₄and N₄₅ to source line SL4; and buried word line BWL5 may be used toselectively couple source ends of NAND strings N₅₀, N₅₁, N₅₂, N₅₃, N₅₄and N₅₅ to source line SL5.

In particular, source side select gate transistors are formed whereburied word lines BWL0, BWL1, . . . , BWL5 and source lines SL0, SL1, .. . , SL5 cross active regions AR0, AR1, . . . , AR5, and where NANDstring contacts 134 are coupled to active regions AR0, AR1, . . . , AR5.For example, FIGS. 2H-2I depict a close-up view of region 160 a of FIG.2C, depicting source side select gate transistor SGS₀₀ formed whereburied word line BWL0 and source line SL0 cross active region AR0, andNAND string contact 134 of vertical NAND string N₀₀ is coupled to activeregion AR0. As is known in the art, buried word line BWL0 may beselectively biased to form a conducting channel 162 between source lineSL0 (e.g., the “drain” of source side select gate transistor SGS₀₀) andNAND string contact 134 of vertical NAND string N₀₀ (the “source” ofsource side select gate transistor SGS₀₀).

In this regard, buried word line BWL0 may be used to selectively couplethe “source” end of NAND string N₀₀ to source line SL0. Similar sourceside select gate transistors are formed for each of NAND strings N₀₀,N₀₁, . . . , N₅₅. Thus, buried word lines BWL0, BWL1, . . . , BWL5 maybe used as SGS selector devices to selectively couple the source ends ofNAND strings N₀₀, N₀₁, . . . , N₅₅ to one of source lines SL0, SL1, . .. , SL5.

Another embodiment of a 3D NAND non-volatile memory device with buriedword line selectors is shown in FIGS. 3A-3I. FIG. 3A illustrates aperspective view of a portion of 3D NAND non-volatile memory device 100b. FIG. 3B illustrates a top view of 3D NAND non-volatile memory device100 b, FIGS. 3C-3E illustrate various cross-sectional views of 3D NANDnon-volatile memory device 100 b, and FIGS. 3F-3I illustrate close-upcross-sectional views of portions of 3D NAND non-volatile memory device100 b. 3D NAND non-volatile memory device 100 b includes source linesSL0, SL1, . . . , SL5, vertical NAND strings N₀₀, N₀₁, . . . , N₅₅, andbit lines BL0, BL1, . . . , BL5 disposed above a surface 110 of asubstrate 112, and a first set of buried word lines BWL0, BWL1, . . . ,BWL5 disposed below surface 110 of substrate 112. In an embodiment,source lines SL0, SL1, . . . , SL5 are connected to one another at acommon source node (not shown).

As shown in FIG. 3C, in an embodiment, 3D NAND non-volatile memorydevice 100 b also includes a second set of buried word lines BWL0′,BWL1′, . . . , BWL5′ disposed below surface 110 of substrate 112. Secondset of buried word lines BWL0′, BWL1′, . . . , BWL5′ are interspersedbetween adjacent first set of buried word lines BWL0, BWL1, . . . , BWL.In an embodiment, first set of buried word lines BWL0′, BWL1′, . . . ,BWL5′ are “active” buried word lines, whereas a second set of buriedword lines BWL0′, BWL1′, . . . , BWL5′ are “passive” buried word linesand provide isolation between adjacent first set of buried word linesBWL0, BWL1, . . . , BWL.

Substrate 112 also includes active regions AR0, AR1, . . . , AR5. Inthis embodiment, source lines SL0, SL1, . . . , SL5 are disposed aboveand are parallel to active regions AR0, AR1, . . . , AR5, and bit linesBL0, BL1, . . . , BL5 are disposed above and are parallel to buried wordlines BWL0, BWL1, . . . , BWL5. In addition, source lines SL0, SL1, . .. , SL5 are perpendicular to buried word lines BWL0, BWL1, . . . , BWL5.In other embodiments, other configurations are possible. For example,active regions AR0, AR1, . . . , AR5 may be disposed at an anglerelative to source lines SL0, SL1, . . . , SL5, buried word lines BWL0,BWL1, . . . , BWL5, and bit lines BL0, BL1, . . . , BL5.

Each of vertical NAND strings N₀₀, N₀₁, . . . , N₅₅ includes columnsC₀₀, C₀₁, . . . , C₅₅, respectively, and alternating conductive layersand dielectric layers. Conductive layers include select gate layers SG0,SG1, . . . , SG5 and word line layers WL0, WL1, . . . , WL7. Forexample, vertical NAND string N₀₀ includes select gate layer SG0 andword line layers WL0, WL1, . . . , WL7, each separated from adjacentconductive layers by dielectric material layers (e.g., SiO₂ or otherdielectric material) (not shown in FIGS. 3C-3E). Likewise, NAND stringN₁₀ includes select gate layer SG1 and word line layers WL0, WL1, . . ., WL7, each separated from adjacent conductive layers by dielectricmaterial layers (not shown in FIGS. 3C-3E). In an embodiment, word linelayers WL0, WL1, . . . , WL7 are perpendicular to source lines SL0, SL1,. . . , SL5.

FIGS. 3F-3G depict a close-up view of region 116 b of column C₀₀ of FIG.3C, showing a source side select gate transistor SGS₀₀ and a memory cellMC₀₇. Region 116 b shows portions of dielectric material layers D0, D1and D2, select gate layer SG0, and word line layer WL7. Each of columnsC₀₀, C₀₁, . . . , C₅₅, includes a number of layers which are depositedalong the sidewalls of the column. These layers may includeoxide-nitride-oxide and polysilicon layers which may be deposited (e.g.,using atomic layer deposition or other technique). For example, a blockoxide (BOX) can be deposited as layer 118, a nitride such as SiN as acharge trapping layer (CTL) can be deposited as layer 120, a tunneloxide (TNL) can be deposited as layer 122, a polysilicon body or channel(CH) can be deposited as layer 124, and a core filler dielectric can bedeposited as region 126. Additional memory cells are similarly formedthroughout columns C₀₀, C₀₁, . . . , C₅₅. In the embodiment shown inFIGS. 3A-3G, each of columns C₀₀, C₀₁, . . . , C₅₅ has a hollow cylindershape. In other embodiments, each of columns C₀₀, C₀₁, . . . , C₅₅ mayhave a solid, rod shape.

Referring again to FIGS. 3A-3E, buried word lines BWL0, BWL1, . . . ,BWL5 are disposed in dielectric trenches 130 (e.g., SiO₂ STI trenches)formed in a doped well 132 (e.g., p-well) in substrate 112. Persons ofordinary skill in the art will understand that three-dimensional NANDdevice 100 b may include more or fewer source lines SL0, SL1, . . . ,SL5, vertical NAND strings N₀₀, N₀₁, . . . , N₅₅, bit lines BL0, BL1, .. . , BL5, and buried word lines BWL0, BWL1, . . . , BWL5 than depictedin FIGS. 3A-3E.

Select gate layers SG0, SG1, . . . , SG5 may be used to selectivelycouple the “source” ends of a NAND strings N₀₀, N₀₁, . . . , N₅₅ to oneof source lines SL0, SL1, . . . , SL5. For example, select gate layerSG0 may be used to selectively couple NAND strings N₀₀, N₀₁, N₀₂, N₀₃,N₀₄ and Nos to source lines SL0, SL1, SL2, SL3, SL4 and SL5,respectively. Likewise, select gate layer SG4 may be used to selectivelycouple NAND strings N₄₀, N₄₁, N₄₂, N₄₃, N₄₄ and N₄₅ to source lines SL0,SL1, SL2, SL3, SL4 and SL5, respectively.

The “drain” ends of NAND strings N₀₀, N₀₁, . . . , N₅₅ are coupled toactive regions AR0, AR1, . . . , AR5 via NAND string contacts 134. Inparticular, NAND strings N₀₀, N₁₀, N₂₀, N₃₀, N₄₀ and N₅₀ are coupled toactive region AR0, NAND strings N₀₁, N₁₁, N₂₁, N₃₁, N₄₁ and N₅₁ arecoupled to active region AR1, NAND strings N₀₂, N₁₂, N₂₂, N₃₂, N₄₂ andN₅₂ are coupled to active region AR2, NAND strings N₀₃, N₁₃, N₂₃, N₃₃,N₄₃ and N₅₃ are coupled to active region AR3, NAND strings N₀₄, N₁₄,N₂₄, N₃₄, N₄₄ and N₅₄ are coupled to active region AR4, and NAND stringsN₀₅, N₁₅, N₂₅, N₃₅, N₄₅ and N₅₅ are coupled to active region AR5. NANDstring contacts 134 are separated from one another and from bit linesBL0, BL1, . . . , BL5 by dielectric material 136 (e.g., SiO₂ or otherdielectric material).

Bit lines BL0, BL1, . . . , BL5 may include a stack of material layers,such as shown in FIGS. 3C-3D. In an embodiment, each of bit lines BL0,BL1, . . . , BL5 include a multi-layer stack of a non-conductivematerial layer 140 (e.g., an etch stop or hard mask material), a firstconductive material layer 142, and a second conductive material layer144. For example, non-conductive material layer 140 may be a nitride,first conductive material layer 142 may be W, and second conductivematerial layer 144 may be highly doped polysilicon (e.g., n+ poly). Inan embodiment, non-conductive material layer 140, first conductivematerial layer 142, and second conductive material layer 144 may beformed on a gate contact layer of a CMOS process. Other materials andprocesses may be used to form bit lines BL0, BL1, . . . , BL5.

Bit lines BL0, BL1, . . . , BL5 form multiple electrical contacts withp-well 132 where bit lines BL0, BL1, . . . , BL5 overlap each of activeregions AR0, AR1, . . . , AR5. For example, referring to FIG. 3C, bitline BL0 forms an electrical contact to p-well 132 at a region 150 wherebit line BL0 overlaps active region AR0, and bit line BL1 forms anelectrical contact to p-well 132 at a region 152 where bit line BL1overlaps active region AR1.

Buried word lines BWL0, BWL1, . . . , BWL5 may be used to selectivelycouple the drain ends of NAND strings N₀₀, N₀₁, . . . , N₅₅ to one ofbit lines BL0, BL1, . . . , BL5. In an embodiment, buried word line BWL0may be used to selectively couple drain ends of NAND strings N₀₀, N₀₁,N₀₂, N₀₃, N₀₄ and N₀₅ to bit line BL0; buried word line BWL1 may be usedto selectively couple drain ends of NAND strings N₁₀, N₁₁, N₁₂, N₁₃, N₁₄and N₁₅ to bit line BL1; buried word line BWL2 may be used toselectively couple drain ends of NAND strings N₂₀, N₂₁, N₂₂, N₂₃, N₂₄and N₂₅ to bit line BL2; buried word line BWL3 may be used toselectively couple drain ends of NAND strings N₃₀, N₃₁, N₃₂, N₃₃, N₃₄and N₃₅ to bit line BL3; buried word line BWL4 may be used toselectively couple drain ends of NAND strings N₄₀, N₄₁, N₄₂, N₄₃, N₄₄and N₄₅ to bit line BL4; and buried word line BWL5 may be used toselectively couple drain ends of NAND strings N₅₀, N₅₁, N₅₂, N₅₃, N₅₄and N₅₅ to bit line BL5.

In particular, drain side select gate transistors are formed whereburied word lines BWL0, BWL1, . . . , BWL5 and bit lines BL0, BL1, . . ., BL5 cross active regions AR0, AR1, . . . , AR5, and where NAND stringcontacts 134 are coupled to active regions AR0, AR1, . . . , AR5. Forexample, FIGS. 3H-3I depict a close-up view of region 160 b of FIG. 3C,depicting drain side select gate transistor SGD₅₀ formed where buriedword line BWL5 and bit line BL5 cross active region AR0, and NAND stringcontact 134 of vertical NAND string N₅₀ is coupled to active region AR0.As is known in the art, buried word line BWL5 may be selectively biasedto form a conducting channel 162 between bit line BL5 (e.g., the “drain”of drain side select gate transistor SGD₅₀) and NAND string contact 134of vertical NAND string N₀₀ (the “source” of drain side select gatetransistor SGD₅₀).

In this regard, buried word line BWL5 may be used to selectively couplethe “drain” end of NAND string N₅₀ to bit line BL5. Similar drain sideselect gate transistors are formed for each of NAND strings N₀₀, N₀₁, .. . , N₅₅. Thus, buried word lines BWL0, BWL1, . . . , BWL5 may be usedas SGD selector devices to selectively couple the drain ends of NANDstrings N₀₀, N₀₁, . . . , N₅₅ to one of bit lines BL0, BL1, . . . , BL5.

In an embodiment, buried word lines BWL0, BWL1, . . . , BWL5 may beimplemented using similar techniques used to fabricate buried word linesin a DRAM process. In some embodiments, 3D NAND non-volatile memoryarrays may be combined with DRAM memory arrays on a single substrate,and fabricated using a single integrated circuit fabrication process.

For example, referring to FIG. 4A, a device 200 may include a 3D NANDarray 202 and a DRAM array 204 integrated on a single substrate 206,both fabricated using a single integrated circuit fabrication process.NAND array 202 may include 3D NAND memory devices, such as 3D NANDnon-volatile memory devices 100 a or 100 b, described above. In such anintegrated device, buried word lines may be used as SGS or SGD selectordevices in 3D NAND array 202, and may be used as DRAM cell transistorsin DRAM array 204.

FIG. 4B illustrates a cross-sectional view of device 200, including 3DNAND array 202 and DRAM array 204 integrated on a single substrate 206.For simplicity, 3D NAND array 202 is depicted including three bit linesBL0, BL1 and BL2 and three select gate layers SG0, SG1, and SG2, andDRAM array 204 is depicted including three bit lines BLD0, BLD1 and BLD2and three select gate layers SG0D, SG1D, SG2D. Persons of ordinary skillin the art will understand that 3D NAND array 202 and DRAM array 204each may have more or fewer than three bit lines and more or fewer thanthree select gate layers.

As shown in FIG. 4B, columns CD₀₀, CD₀₁ and CD₀₂ are not connected atthe top to any of the source lines (e.g., SL0). Although not depicted inFIG. 4B, in DRAM array 204, word lines (e.g., WLD0, WLD1, . . . , WLD7)and select gate layers (e.g., SG0D, SG1D, SG2D) may be electricallyconnected and used as a common DRAM plate electrode. In an embodiment,word lines WLD0, WLD1, . . . , WLD7 and select gate layers SG0D, SG1D,SG2D may be biased to a plate voltage (e.g., 3-5 V) to invert the DRAMcell node (e.g., the NAND channel 124 in columns CD₀₀, CD₀₁ and CD₀₂).In addition, bit lines BLD0, BLD1 and BLD2 may be biased to a bit linevoltage (e.g., 0-1V).

In 2D NAND memory devices, the p-well substrate is biased at a highvoltage to erase the storage elements (memory cells). In contrast, a 3Dstacked non-volatile memory device such as 3D NAND non-volatile memorydevices 100 a and 100 b do not have a substrate. One approach to erasingis to generate gate induced drain leakage (GIDL) current to charge upthe channel, raise the channel potential to an erase voltage, andmaintain this channel potential during erase. In previously known 3DNAND stacked non-volatile memory devices, the select gate transistorsSGD and SGS are used to generate a sufficient amount of GIDL current tocharge up the floating body of the NAND string in a reasonable timeframe.

In an embodiment, bottom word lines of 3D NAND non-volatile memorydevices 100 a or 100 b may be used as hole generators using GIDL. Forexample, word line WL0 of 3D NAND non-volatile memory devices 100 a or100 b may be biased under conditions which generate GIDL.

GIDL hole current for erase is needed for charge trap layer cells, andmay be generated either from the top of a 3D NAND non-volatile memorydevice via biasing the BL or SL to Verase, and the top SGD or top SGSgate to a voltage Verase-Vgidl, where e.g., Verase=20V and Vgidl=8V. Incase of floating gate cells, no GIDL hole generation is needed.Accordingly GIDL can be generated from the bottom of 3D NANDnon-volatile memory devices 100 a or 100 b by biasing p-well 132 toVerase, and floating SGS or SGD gate as well as SL or BL. In this lattercase, the bottom n+ junction is charged up to Verase via the p-well 132bias. GIDL is generated from this n+ junction if the first bottom wordline (e.g., WL0) is biased to, for example Verase-Vgidl=12V.

FIG. 5A is a perspective view of a 3D stacked non-volatile memory device1000. Memory device 1000 includes a substrate 1002, which includesexample blocks BLK0 and BLK1 of memory cells and a peripheral area 1004with circuitry for use by blocks BLK0 and BLK1. Substrate 1002 also mayinclude circuitry (not shown) under blocks BLK0 and BLK1, and one ormore lower metal layers which are patterned in conductive paths (notshown) to carry signals of the circuitry. Blocks BLK0 and BLK1 areformed in an intermediate region 1006 of memory device 1000. In an upperregion 1008 of memory device 1000, one or more upper metal layers arepatterned in conductive paths (not shown) to carry signals of thecircuitry. Each of blocks BLK0 and BLK1 includes an array of 3D NANDstacked memory cells (e.g., 3D NAND non-volatile memory devices 100 aand/or 100 b of FIGS. 2A-3I), where alternating levels of the stackrepresent word lines. Although two blocks BLK0 and BLK1 are depicted asan example, additional blocks can be used, extending in the x- and/ory-directions.

In example embodiments, the x-direction represents a direction in whichsignal paths to word lines extend in the one or more upper metal layers(a word line direction), and the y-direction represents a direction inwhich signal paths to bit lines extend in the one or more upper metallayers (a bit line direction). The z-direction represents a height ofmemory device 1000.

FIG. 5B is a functional block diagram of memory device 1000 of FIG. 5A.Memory device 1000 may include one or more memory die 1010. Memory die1010 includes a 3D memory array 1012 of memory cells, e.g., includingblocks BLK0 and BLK1, a control circuit 1014, and read/write circuits1016. Memory array 1012 is addressable by word lines via a row decoder1018 and by bit lines via a column decoder 1020. Read/write circuits1016 include multiple sense blocks 1022 (sensing circuitry) and allow apage of memory cells to be read or programmed in parallel. Typically acontroller 1024 is included in the same memory device 1000 (e.g., aremovable storage card) as memory die 1010. Commands and data aretransferred between a host and controller 1024 via lines 1026 andbetween controller 1024 and memory die 1010 via lines 1028.

Control circuit 1014 cooperates with read/write circuits 1016 to performmemory operations on memory array 1012, and includes a state machine1030, an on-chip address decoder 1032, and a power control module 1034.State machine 1030 provides chip-level control of memory operations.On-chip address decoder 1032 provides an address interface between thatused by the host or a memory controller to the hardware address used byrow decoder 1018 and column decoder 1020. Power control module 1034controls the power and voltages supplied to the word lines and bit linesduring memory operations. Power control module 1034 may include driversfor WLLs and WLL portions, drain- and source-side select gate drivers(referring, e.g., to drain- and source-sides or ends of a string ofmemory cells such as a NAND string, for instance) and source lines.Sense blocks 1022 may include bit line drivers, in one approach.

In some implementations, some of the components can be combined. Invarious designs, one or more of the components (alone or incombination), other than memory array 1012, can be thought of as atleast one control circuit. For example, a control circuit may includeany one of, or a combination of, control circuitry 1014, row decoder1018, column decoder 1020, sense blocks 1030, read/write circuits 1016,and controller 1024, and so forth.

In another approach, a non-volatile memory system uses dual row/columndecoders and read/write circuits. Access to memory array 1012 by thevarious peripheral circuits is implemented in a symmetric fashion, onopposite sides of the array, so that the densities of access lines andcircuitry on each side are reduced by half Thus, the row decoder issplit into two row decoders and the column decoder into two columndecoders. Similarly, the read/write circuits are split into read/writecircuits connecting to bit lines from the bottom and read/write circuitsconnecting to bit lines from the top of memory array 1012. In this way,the density of the read/write modules is reduced by one half Other typesof non-volatile memory in addition to NAND flash memory can also beused.

Accordingly, it can be seen that one embodiment includes athree-dimensional stacked non-volatile memory device including a stackincluding alternating word line and dielectric layers and a plurality ofNAND strings of memory cells formed in memory holes which extend throughthe layers, each memory cell including a control gate formed by one ofthe word line layers, and multiple selector devices, each selectordevice coupled to an end of a corresponding NAND string. The NANDstrings are disposed above a substrate, and the selector devices aredisposed in the substrate.

Another embodiment includes a method of forming a three-dimensionalstacked non-volatile memory device. The method includes forming a stackincluding alternating word line and dielectric layers and a plurality ofNAND strings of memory cells formed in memory holes which extend throughthe layers, each memory cell including a control gate formed by one ofthe word line layers, forming a plurality of selector devices, eachselector device coupled to an end of a corresponding NAND string,forming the NAND strings above a substrate, and forming the selectordevices in the substrate.

Still another embodiment includes a device that includes athree-dimensional NAND stacked non-volatile memory array, and a DRAMmemory array. The three-dimensional NAND stacked non-volatile memoryarray and the DRAM memory array are integrated on a single substrate.

Still another embodiment includes a method that includes forming athree-dimensional NAND stacked non-volatile memory array on a substrate,and forming a DRAM memory array on the substrate. The three-dimensionalNAND stacked non-volatile memory array and the DRAM memory array areformed using a single integrated circuit fabrication process.

The foregoing detailed description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or belimited to the precise form disclosed. Many modifications and variationsare possible in light of the above description. The describedembodiments were chosen to best explain the principles of the technologyand its practical application, to thereby enable others skilled in theart to best utilize the technology in various embodiments and withvarious modifications as are suited to the particular use contemplated.The scope of the technology is defined by the claims appended hereto.

The invention claimed is:
 1. A three-dimensional stacked non-volatilememory device comprising: a stack comprising alternating word line anddielectric layers and a plurality of NAND strings of memory cells formedin memory holes which extend through the layers, each memory cellcomprising a control gate formed by one of the word line layers; and aplurality of selector devices, each selector device coupled to an end ofa corresponding NAND string, wherein the NAND strings are disposed abovea substrate, and the selector devices are disposed in the substrate. 2.The three-dimensional stacked non-volatile memory device of claim 1,wherein each selector device comprises a buried word line.
 3. Thethree-dimensional stacked non-volatile memory device of claim 2, whereineach buried word line is disposed in a corresponding trench in thesubstrate.
 4. The three-dimensional stacked non-volatile memory deviceof claim 1, further comprising a plurality of select gate transistors,each select gate transistor comprising a corresponding one of theselector devices.
 5. The three-dimensional stacked non-volatile memorydevice of claim 4, wherein each select gate transistor comprises asource-side select gate transistor or a drain-side select gatetransistor.
 6. The three-dimensional stacked non-volatile memory deviceof claim 1, further comprising a plurality of source lines, wherein eachselector device is coupled to a corresponding one of the source lines.7. The three-dimensional stacked non-volatile memory device of claim 6,wherein the source lines each comprise a multi-layer stack of anon-conductive material layer, a first conductive material layer, and asecond conductive material layer.
 8. The three-dimensional stackednon-volatile memory device of claim 1, further comprising a plurality ofbit lines, wherein each selector device is coupled to a correspondingone of the bit lines.
 9. The three-dimensional stacked non-volatilememory device of claim 8, wherein the bit lines each comprise amulti-layer stack of a non-conductive material layer, a first conductivematerial layer, and a second conductive material layer.